This invention relates to a portable-type bubble memory apparatus, and more particularly to a bubble memory apparatus so adapted as to prevent the loss of information stored in the bubble memory even if the bubble memory cassette is inadvertently or abruptly withdrawn from a portable cassette adapter.
Numerical control devices for controlling machine tools have recently been adapted to employ bubble memories rather than paper tapes as the storage medium. These bubble memories are magnetically driven and therefore require comparatively expensive equipment to enable the bubble memory element to be used as the storage medium. Such equipment includes peripheral circuitry composed of a driver for driving a coil located within the bubble memory element and for driving a variety of gates, a sense amplifier for reading outputs, a function driver and a timing circuit, as well as a control circuit for controlling data read and write operations. The apparatus that utilizes the conventional bubble memory is therefore provided with (1) a bubble memory cassette having a bubble memory element, and (2) a portable cassette adapter which contains the control circuit, the peripheral circuitry and a holder for holding the inserted bubble memory cassette and for establishing electrical contact with the cassette, the cassette adapter responding to commands from the numerical control device to read information from the bubble memory cassette and to write information into memory. To reduce costs, it is so arranged that a single portable cassette adapter can be used in common by individual numerical control devices, the adapter being designed in such a manner that bubble memory cassettes storing different kinds of numerical control information can be exchanged merely by ejecting one cassette and replacing it with another.
In the bubble memory cassette apparatus of the above type, the bubble memory cassette can be withdrawn from the portable cassette adapter at any time. However, since the bubble memory cassette accommodates neither the coil driver for generating a revolving magnetic field for bubble transfer, nor a back-up power source which renders the coil driver operative for a predetermined period of time, sudden withdrawal of the bubble memory cassette during a shift of bubbles within the major and minor loops in the bubble memory may cause a shifting bubble to stop short somewhere within a loop, so that the data stored in memory may not be preserved correctly.